Size Evolution of Photoabsorption Spectra of Small Clusters: A Computational Study

Abstract

Photoabsorption spectra of ${\mathrm{He}}_N^{+}$ clusters, N=5-9, have been calculated using a diatomics-in-molecules like electronic structure model and a path-integral Monte Carlo sampling method. A qualitative change in the calculated spectra has been observed at N=9, which has been interpreted in terms of a structural transformation in the clusters consisting in a transition from trimer-like ionic cores observed for N≤7 to dimer-like ionic cores prevailing in ${\mathrm{He}}_9^{+}$ through an intermediate state (comparable abundances of both types of ionic cores) observed in ${\mathrm{He}}_8^{+}$ . The calculated spectra have been thoroughly compared with an earlier calculation on ${\mathrm{He}}_3^{+}$ , ${\mathrm{He}}_4^{+}$ , and ${\mathrm{He}}_{10}^{+}$ reported from our group and data available for the same cluster sizes from an experiment.

Keywords: charged helium clusters; diatomics-in-molecules; geometric structure; path-integral Monte Carlo; photoabsorption spectra.